The present invention relates to a liquid jetting head, a liquid jetting apparatus, and a method of manufacturing the liquid jetting head.
In recent years, in the field of image processing and the like, there has been an increasing need for transition from monochromic hard copying to color hard copying. Hitherto, there have been proposed such color copying systems as sublimation type thermal transfer system, fusion thermal transfer system, ink jet system, electrophotographic system, and thermal development silver salt system.
Among these systems, the ink jet system is a system in which droplets of a recording liquid (ink) are let fly from nozzles provided in a printer head serving as a liquid jetting head and are adhered to a recording object, thereby forming dots on the recording object. The ink jet system can output high-quality images while using a simple configuration. The ink jet system is classified into electrostatic attraction system, continuous vibration generating system (piezo system), and thermal system, according to the differences in the system for flying the ink droplets from the nozzles.
Among the above systems, the thermal system is a system in which a bubble is generated by local heating of the ink, and the ink is pushed out through a nozzle by the bubble, to fly onto the printing object. The thermal system can print color images while using a simple configuration.
In a printer head based on such a thermal system includes heating elements for heating the ink(s) which are integrally formed on a semiconductor substrate together with a driving circuit composed of a logic integrated circuit for driving the heating elements. This ensures that, in this type of printer head, the heating elements are arranged in a high density so that they can be securely driven.
In other words, in the printer of the thermal system, the heating elements must be arranged in a high density, in order to obtain print results with high image quality. Specifically, in order to obtain print results corresponding to 600 DPI, for example, the heating elements must be arranged at an interval of 42.333 μm. It is extremely difficult to arrange individual driving elements for the heating elements arranged in such a high density. In view of this, in the printer head, switching transistors or the like are formed on the semiconductor substrate and connected to the corresponding heating elements by the integrated circuit technology, and, further, the switching transistors are driven by a driving circuit similarly formed on the semiconductor substrate, whereby the heating elements can be driven easily and securely.
To be more specific, FIG. 11 is a sectional view showing the configuration in the vicinity of a switching transistor in this type of printer head. The printer head 1 has a structure in which device separation regions for insulatingly separating MOS (Metal Oxide Semiconductor) type field effect transistors is formed on a silicon substrate 2, and thereafter the MOS type transistors 3 and the like are formed between the device separation regions, whereby switching transistors served to drive heating elements and a driving circuit for driving the switching transistors are composed of the MOS type transistors formed by a semiconductor production process.
Subsequently, an inter-layer insulation film for insulating the MOS type transistors 3 and the like are laminated, the inter-layer insulation film is then provided with openings (contact holes), and the first layer of wiring pattern 4 and the heating elements are sequentially formed. Here, the heating elements are formed of tantalum (Ta), tantalum nitride (TaNX), or tantalum-aluminum (TaAl). Subsequently, an inter-layer insulation film 5 for insulating the first layer of wiring pattern 4 and the following second layer of wiring pattern from each other and the like are laminated, the inter-layer insulation film 5 is then provided with openings (via holes) to form the second layer of wiring pattern 6, and the heating elements are connected to the MOS type transistors 3 through the two layers of wiring patterns 4, 6. Followingly, an insulating protective layer 7 of silicon nitride (Si3N4) and an anti-cavitation layer of β-tantalum are sequentially formed on the heating elements.
In the manufacture of the printer head 1, a photosensitive resin material is then applied to the whole surface of the substrate thus provided with the heating elements and the like, and surplus portions of the applied photosensitive resin are removed by exposure and development steps. Further, a nozzle plate formed of a nickel-cobalt alloy (Ni—Co) is adhered to the upper layer of the assembly, whereby ink liquid chambers, ink conduits for introducing the ink into the ink liquid chambers, and nozzles are formed. The printer head 1 has such a configuration that pulses of voltage are impressed on the heating elements by the MOS type transistors 3, to drive the heating elements, thereby letting ink droplets to fly out.
In the printer head 1 configured as above, mere lamination of the component members cannot obviate the generation of steps due to the wiring pattern 4 and the like in the surface of the insulating protective layer 7, and the steps thus generated leads to the development of steps also in the surface of the resin layer formed on the upper side of the insulating protective layer 7, so that a gap is generated between the nozzle sheet adhered to the resin layer and the surface of the resin layer. In the printer head 1, the generation of such a gap may deteriorate the adhesion between the resin layer and the nozzle sheet.
In relation to this, it is considered that the technique disclosed in U.S. Pat. No. 6,450,622 may be applied, to eliminate such steps by an SOG (Spin On Glass) film, whereby the nozzle sheet can be held sufficiently firmly. Here, the SOG film is formed by a method in which a coating type insulating material containing a siloxane component in an alcohol component serving as a solvent is applied to the substrate surface by the spin coating process, to be built up so as to fill the stepped portions, and then the whole surface of the thus built-up insulating material film is etched back by an etching back process using wet etching or dry etching.
However, if the steps are eliminated merely by the SOG film, the problem of deterioration of the heating elements due to the driving of the heating elements would be generated in the printer head 1. Specifically, when the heating elements were driven in the condition where the ink is not held in the ink liquid chambers (so-called no-load driving), it was confirmed that the resistance of the heating elements in the printer head 1 was raised conspicuously and that the surface of the anti-cavitation layer was turned black, as the relevant regions are indicated by broken lines a in FIG. 12.
The example shown in FIG. 12 corresponds to the case where rectangular resistor films 8, 9 were formed side by side with a predetermined spacing therebetween, one-side ends of the resistor films 8, 9 were connected to each other by a wiring pattern 6a, and a driving voltage was applied to the other-side ends of the resistor films 8, 9 by wiring patterns 6b, 6c, whereby a heating element was composed of the series connection of the resistor films 8, 9. In addition, the heating element portion corresponds to the case where the anti-cavitation layer having a film thickness of 200 nm, the insulating protective layer 7 having a film thickness of 300 nm, the heating element having a film thickness of 100 nm, the inter-layer insulation film 5 composed of a silicon oxide film 400 nm in film thickness, the SOG film 450 nm in film thickness, a silicon nitride film 250 nm in film thickness and a silicon oxide film 1130 nm in film thickness, and the like were formed, in this order as viewed from the ink liquid chamber side. In this example, the heating element was driven by a rated driving power of 0.8 W.
Detailed investigation of this point showed that the heat due to the driving of the heating element is transferred to the SOG film, and the SOG film itself is decomposed by the heat, or the solvent component left in the SOG film is librated, whereby the heating element is oxidized or carbonized, leading to a marked increase in resistance.
Incidentally, it is considered that, in relation to the heating element forming location, the SOG film may be selectively removed by, for example, wet etching, whereby the above-mentioned deterioration of the heating element can be prevented. However, the wet etching of the SOG film renders the so-called overhang conspicuous, and the residue upon formation of the second layer of wiring pattern is left at the portion of the overhang, with the result that the wiring pattern or the like is short-circuited due to the residue.